1. Field of the Invention
The present invention relates to a piezoelectric switch having no moving parts, particularly for use in keyboards. The switch includes a piezoelectric ceramic, having both contacts on one side, and an electronic circuit using an anisotropic elastomer as a conductive linking media to convert mechanical force to an electrical signal.
2. The Prior Art
Known switches of the type used in keyboards, for example, computer keyboards or telephone keypads, are constructed of a number of moving parts. These type of switches frequently break down due to the fragile nature of the parts and must be frequently cleaned to prevent damage due to dust and dirt.
U.S. Pat. No. 4,737,767 discloses a keyboard, shown in FIG. 1, which consists of a number of keyswitches having no moving parts so as to eliminate the previously mentioned disadvantages. The keyboard casing 2 is formed of a hard non-insulating material. A layer 6 of epoxy is applied to the casing 2 which transmits the force imposed on the different keyboard areas 4. A number of thin electrically insulating sheets 8 are attached to the hard non-insulating material at individual areas 4 in the housing representing the characters on the keyboard. A plurality of thin electrically conductive sheets 10 are attached to the thin insulating sheets 8, which together constitute printed curcuit segments. A number of ceramic piezoelectric crystals 14 are attached to the individual conductive sheets 10 with a small amount of epoxy 12 and the conductive sheets 10 are thus electrically coupled to the housing. In this way, one surface of the crystal is common to the individual area of the housing and when the keyboard area is depressed, the crystal becomes stressed and causes an electrical signal to be generated on the ungrounded surface of the crystal. Leads 16 are attached to the ungrounded surface of the crystals 14 and carry the electrical signal to the outside world. An insulating layer 18 overlays the crystals 14 and the conductive sheets 10.
Switches of this type have a number of disadvantages. They are complicated in structure, and thus are difficult and costly to manufacture. Because the individual parts must be soldered or glued together, they are difficult to replace or repair. In addition to the many layers of this switch, the ceramic has contacts on opposite sides thereof, thus making electrical contact therewith difficult and complex. Furthermore, in order to construct an entire keyboard, it is necessary to use individual ceramics and corresponding insulating and conductive layers. Additionally, the printed circuit is located between the crystal and the hard insulating material, thus resulting in the possibility of damage to the printed circuit.
The use of piezoelectric ceramics having both contacts on one side in electric circuits is known. Examples of patents disclosing such an arrangement are as follows.
______________________________________ U.S. Pat. No. Inventor U.S. Pat. No. Inventor ______________________________________ 3,885,173 Lee 3,390,287 Sonderegger 2,575,819 Laird 2,385,896 Von Beckerath 2,258,967 Bokovoy ______________________________________
Also known is the use of a conductive rubber gasket either as connecting links or in association with a piezoelectric ceramic, examples of which are found in the following patents.
______________________________________ U.S. Pat. No. Inventor U.S. Pat. No. Inventor ______________________________________ 4,532,451 Inoue et al. 4,360,754 Toyoshima et al. 4,322,652 Otsuka 4,267,480 Kanematsu et al. 4,344,662 Dalamangas 4,252,391 Sado et al. 4,221,444 Patrick 4,201,435 Nakamura et al. 3,885,173 Lee ______________________________________
Only Lee discloses the use of a conductive rubber gasket in conjunction with a piezoelectric ceramic having both contacts on one side. Lee is directed to a device for mechanically and electrically coupling an acoustical surface wave device to a housing. The device includes a conducting rubber gasket connected between an acoustical surface wave device and an electronic circuit within a housing. The acoustical surface wave device acts as a filter, which allows information to pass through the circuit when the carrier corresponds with the frequency of the wave. The circuitry connected to the piezoelectric ceramic is for feeding signals to the ceramic, rather than being solely for receiving signals from the ceramic. Thus, the device in Lee cannot function as a key switch as in the present invention.